advice on mortgage:split rates.

I was hoping for some advice. My husband and I are building. We have planning permission. His Father is a builder and is building our house for 215,000. We have savings, permanent jobs and got site as present, so loan to value would be under 80%. We don't know how much to take out on loan. Don't want to take out too little and have to go back. Someone mentioned splitting it over fixed and variable rates so that if we take out too much ,we don't have to draw it all down under variable rate. Is this right? Do we decide how to split it?
If we are looking for 240,000 say, should we tell the lender that build is costing 264,000 (add on 10%) or because we have the site and savings, can we be honest with them?
From looking at rates AIB seem the best. Am I right in thinking this? Any advice welcome as feel a bit swamped!! Thanks.

If I were taking out a mortgage today I'd go with an AIB 50% variable, 50% fixed for 5 years (10 years if I was confident I'd not be moving during that time).

Thanks for that. Is that a package they provide or would I have to ask for that? Do you know what rates are like for it?

AIB rates [broken link removed]. Says you can split [broken link removed]. Don't know what impact 'self-build' might have on mortgage offer.